I was navigating on my Ubuntu 12.10 64bit box through Windows WinSCP with root user and suddenly pressed "delete" "enter" when I was on usr directory. I immediately cancelled the delete process and the mess was already done. I got no python3 interpreter:
-bash: /usr/lib/command-not-found: /usr/bin/python3: invalid interpreter: File or directory not found.
Please, don't say reinstall your system or boot a LiveCD. I can't use dpkg or apt-get to reinstall the packages.